The photochemical conversion of 200-500 nm layers of perhydropolysilazane --(SiH2-NH)n-- (PHPS) in the presence of oxygen into an SiOx network was studied. Different UV sources in the wavelength range of 160-240 nm, that is, 172 nm Xe2* and 222 nm KrCl* excimer, and 185 nm Hg low-pressure (HgLP) lamps were used for these purposes. The role of both ozone and O(1D) as well as of catalytic amounts of tertiary amines in the degradation process of PHPS and the formation of SiOx were studied. In this context, the kinetics of the entire reaction were elucidated and allowed both a continuous and discontinuous process to be established for the production of fully transparent, flexible barrier coatings. Barrier improvement factors (BIFs) of 400 were achieved with one single layer on 23 microm poly(ethyleneterephthalate) (PET), which translated into oxygen transmission rates (OTRs) of 0.20 cm3 m(-2) day(-1) bar(-1). Double layers prepared by this technique allowed the realization of OTRs of or=800.

Microimaging by IR microscopy is applied to the recording of the evolution of the concentration profiles of reactant and product molecules during catalytic reaction, notably during the hydrogenation of benzene to cyclohexane by nickel dispersed within a nanoporous glass. Being defined as the ratio between the reaction rate in the presence of and without diffusion limitation, the effectiveness factors of catalytic reactions were previously determined by deliberately varying the extent of transport limitation by changing a suitably chosen system parameter, such as the particle size and by comparison of the respective reaction rates. With the novel options of microimaging, effectiveness factors become accessible in a single measurement by simply monitoring the distribution of the reactant molecules over the catalyst particles.

Open sesame: Novel cationic RuII complexes with N‐heterocyclic carbene ligands (see structure) have been prepared and used as UV‐triggerable photoinitiators for ring‐opening metathesis polymerization. They may be used for the high‐yield synthesis of bulk polymers as well as for surface functionalization. Laser pulse radiolysis and NMR spectroscopy experiments supported by quantum chemical calculations give insight into the initiation mechanism.

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In a two-dimensional electron gas with a spatially modulated charge density we observe a splitting of the two-dimensional-plasmon dispersion. The charge-density modulation is induced in a metal-oxide-silicon capacitor with a modulated oxide thickness of submicrometer periodicity. The splitting is caused by the superlattice effect of the charge-density modulation on the collective excitation spectrum and depends strongly on the Fourier expansion coefficients of the charge-density profile.PACS numbers: 71.45. Gm, 73.40.Qv In metal-oxide-semiconductor (MOS) structures electrons can be confined in a narrow potential well. These space-charge layers exhibit two-dimensional (2D) electronic properties which can be varied via the gate potential. 1 One interesting aspect of the 2D-MOS system is the possibility to lower the dimensionality by periodic lateral potential variation which may induce superlattice effects. Such lateral potential variation has been shown to arise naturally on high-index surfaces of semiconductors. 2,3 By use of submicron technology we have fabricated MOS structures with a lateral charge-density modulation which leads to new artificially created electronic properties. In these structures we observe that the 2D-plasmon frequency co p , which is known to vary continuously with the square root of the plasmon wave vector q in a 2D system with constant charge density,^8 exhibits gaps and separated plasmon bands.The sample geometry is shown schematically in Fig. 1. We use large-area (diameter, 5 mm) MOS capacitors on /?-Si (100) substrates (substrate resistivity, 20 n/n; peak channel mobility, 3000 cm 2 /V-s). The thickness of the insulating oxide varies with a periodicity a. The linear periodic FIG. 1. Schematic geometry of oxide-modulated MOS capacitors.structures are prepared by holographic lithography and dry etching techniques. Samples with periodicities from 300 to 1200 nm have been fabricated. A thin layer of 3 nm NiCr is evaporated with varying angles onto the structures to achieve a continuous semitransparent gate. If a gate voltage V g is applied between gate and substrate, stripes of different charge density are induced at the semiconductor interface. The widths of the stripes, t\ and t 2 = i a -t u are controlled by the holographically prepared mask, and the oxide thickness d x by the etching process. For the dimensions used here the charge density N si in the regime /, can be approximated by N S i sss € 0X (V g -V tI )/d h / = 1,2. e ox is the static dielectric constant of the oxide. The threshold voltage V t is determined from the onset of the integrated dynamic conductivity, which is measured in the frequency regime 20 cm -1 to 400 cm -1 and averaged over the gate area. This threshold agrees with results from Shubnikov-de Haas and from CV experiments. A difference in the threshold voltages in the regimes t\ and t 2 has not been resolved, and so we take V n = V t2 = V t in the following. The excitation of plasmons is investigated by transmission spectroscopy. We measure the relative chang...